Tolkien, without a doubt. Considering the amount of time he had creating Middle-earth, going on years even after The Hobbit and Lord of the Rings were published, constantly extending and expanding the universe; creating actual made up languages and cultures and extensive histories for particular races in his universe, and continued doing so until the day he died (keep in mind that The Silmarillion wasn't even finished at that time, and there are possibly other works of his that he was working on that are never going to see the light of day), it's remarkable. Tolkien has become known as the master and grandfather of fantasy worldbuilding, it's quite extraordinary.

There's definitely other fandoms that had great worldbuilding for sure, but I think Tolkien set the bar especially in terms of fantasy worldbuilding which became heavily influential for the genre afterwards. So yeah, it's really hard to top that.

His imagination was quite extensive, and he kept adding on and it's really impressive. I adore worldbuilding that has many layers upon layers, which Tolkien clearly did with everything involving Middle-earth. :)

I will concur that Tolkien wins on sheer depth and scale in his worldbuilding- I'm not sure if there's ever been another linguistics professor with that much understanding of how languages work who's tried to write a work of that length.

It's a lot harder to say if his was the most inventive, though, since a lot of the elements in Middle Earth are now basically common stock of fantasy. He did draw heavily from mainly European sources, particularly Wagner's Ring and Norse mythology, so there's that.